These studies are concerned with protein-calorie malnutrition in cebus and squirrel monkeys. Specific studies include (a) the definition of protein, amino acid and energy requirements assessed primarily by following growth, plasma protein and amino acid levels and hematologic indices; (b) to characterize protein and calorie deficiencies alone and in combination; (c) to evaluate the effects of these deficiencies in early life upon physical and behavioral development. X-rays are taken longitudinally to assess skeletal development and a series of observations and tests (habituation, exploratory behavior and learning) are used to assess behavioral development. Special attention is paid to the possible residual effects after rehabilitation. The studies are designed to evaluate the effects of various types of nutritional deprivation and three degrees of social deprivation (isolation in single cases, peer interaction and peer interaction with handling) and the interaction between nutritional and social deprivation; (d) special attention will be devoted to the study of the defect in carbohydrate metabolism which develops in protein-deficient squirrel monkeys. This involves glucose tolerance tests and measurements of insulin growth hormone, cortisol, glucagon and free fatty acids. The severe anemia which develops in young animals; (e) a wide variety of immunological methods will be used to evaluate the effects of various kinds of nutritional deprivation upon the development and response of immune systems.